1. Field of the Invention
The present invention relates to an apparatus for implementing a 3-dimensional virtual sound and method thereof. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for enabling implementation of 3-dimensional (3-D) virtual sound in such a mobile platform failing to be equipped with expensive instruments for the implementation of the 3-dimensional sound as a mobile communication terminal and the like.
2. Discussion of the Related Art
Recently, many efforts are made to the research and development of the 3-D virtual audio technology that can bring about a 3-dimensional sound effect using only a pair of speakers or a headset without employing high-grade equipments in a multimedia device that requires 3-dimensional virtual reality for multimedia contents, CD-ROM title, game player, virtual reality and the like. In the 3-D virtual audio technology, sensibilities of direction, distance, space and the like are formed as if a sound comes from the position where the virtual sound source is located in a manner of establishing a sound source at a specific position via headset or speaker to enable a user to listen to the sound.
In most of the 3-D virtual audio technologies, a head related transfer function (hereinafter abbreviated HRTF) is used to give a virtual sound effect to a speaker or headset.
The virtual sound effect is to bring about an effect such that a sound source is located at a specific position in a 3-dimensional virtual space. And, the virtual sound effect is achieved by filtering the sound stream from a mono sound source with head related transfer function (HRTF).
The head related transfer function (HRTF) is measured in an anechoic chamber by targeting on a dummy head. In particular, Pseudo-random binary sequences are output from a plurality of speakers that are spherically deployed at various angles centering on the dummy head within the anechoic chamber, respectively and the received signals are then measured by microphones provided to both ears of the dummy head to compute the transfer functions of the acoustic paths. And, this transfer function is called a head related transfer function (HRTF).
A method of seeking a head related transfer function (HRTF) is explained in detail as follows.
First of all, elevations and azimuths are subdivided into predetermined intervals centering on a dummy head, respectively. Speakers are placed at the subdivided angles, e.g., 10° each, respectively. Pseudo-random binary sequences are output from a speaker placed at each position on this grid of subdivided angles. Signals arriving at right and left microphones, placed in the ears of the dummy head, are then measured. The impulse responses and hence the transfer functions of the acoustic paths from the speaker to the left and right ear are then computed. An unmeasured head related transfer function in a discontinuous space can be found by interpolation between neighbor head related transfer functions. Hence, a head related transfer function database can be established in the above manner.
As mentioned in the foregoing description, the virtual sound effect is to bring about an effect that a sound source seems to be located at a specific position in a 3-D virtual space.
The 3-D virtual audio technology can generate an effect that a sound can be sensed at a fixed specific position and another effect that a sound moves away from one position into another position. In particular, the static or positioned sound generation can be achieved by performing a filtering operation using a head related transfer function at a corresponding position of the audio stream from a mono sound source. And, a dynamic or moving sound generation can be achieved by performing filtering operations, in a continuous manner, using a set of Head-related functions (corresponding to the different points on the trajectory of the moving sound source) with the audio stream from a mono sound source.
Since the above-explained 3-D virtual audio technology needs storage space for storing a large database of head related transfer functions to generate the static (positioned) and dynamic (moving) sounds and also requires a lot of computations for the execution of the filtering operation on the signal from the mono sound source with the head related transfer function, high-performance hardware (HW) and software (SW) equipments are necessary for real-time implementation.
Besides, in applying the 3-D virtual audio technology to movies, virtual realities, games and the like, which need the implementation of the virtual 3-D sound for multiple moving sounds, the following problems are brought about.
First of all, if the HRTFs are directly approximated using low-order IIR (infinite impulse response) filters, unique for each position in 3-dimensional space (as done in existing proposals due to the ability of IIR filters to model HRTFs with lower computational complexity compared to the FIR (finite impulse response) filters), in order to simulate a mono-sound source moving from one position to another using the 3-D virtual audio technology, a switching from one IIR (infinite impulse response) filter corresponding to the initial position of the sound source to another IIR filter corresponding to a next position in the sound source trajectory is needed.
Yet, while the sound source makes a transition from one position in space to another, switching between two IIR filters modeling HRTFs can make the system unstable and may give rise to audible “clicking” noise while making a transition from one filter to the other.
Secondly, if the HRTF model is unique to a location in space, as exist in many state-of-art systems, simulation of a set of sound sources occupying different positions in space requires a set of filters modeling the HRTFs corresponding to the positions of the sound sources in the auditory space. To simulate N sound sources, N filters need to be operational in real-time. Hence, complexity scales up linearly as the number of sound sources in the set increases. In particular, to give the 3-D sound effect according to the multiple moving sounds to multimedia contents such as movies, virtual realities, games and the like, high-performance hardware and software equipments capable of providing a large-scale storage space and real-time operation capability are needed.